Industrial-scale ammonium sulfate production

Abstract

The present invention provides a method for producing crystalline ammonium sulfate, comprising the steps of: conducting a Beckmann rearrangement reaction; neutralizing the Beckmann rearrangement reaction mixture; separating a first aqueous ammonium sulfate phase and an aqueous ε-caprolactam phase; introducing the first ammonium sulfate phase into a first evaporative crystallization section to obtain crystalline ammonium sulfate; discharging a mother liquor enriched in organic components from the first evaporative crystallization section; extracting the aqueous ε-caprolactam phase to obtain an extracted ε-caprolactam phase and a second aqueous ammonium sulfate phase; and discharging the mother liquor and / or the second aqueous ammonium sulfate phase discharged from the first evaporative crystallization section to the second evaporative crystallization section, wherein the evaporative crystallization is performed to produce a three-phase system. At least a liquid oily phase is recovered from the three-phase system. The present invention also provides a plant suitable for carrying out the method of the present invention, and the crystalline ammonium sulfate and liquid oily phase obtained by the method of the present invention.

Description

Detailed Description of the Invention

[0001] [Technical Field of the Invention] The present invention relates to an industrial-scale continuous process for producing crystalline ammonium sulfate from an aqueous stream containing organic components, said aqueous stream originating from an ε-caprolactam production process.

[0002] [Background of the Invention] Ammonium sulfate is useful as a fertilizer providing nitrogen and sulfur, for example for use in agriculture, horticulture or forestry, and this type of use often utilizes it in crystalline form. The particle size distribution of ammonium sulfate crystals plays a significant role for easy use as a fertilizer. Generally, in the fertilizer industry, large crystals are desired and are easier to handle. Crystals with a relatively large average crystal size can be used in highly commercially valuable fertilizer formulations and thus have a higher economic value than smaller crystals. Ammonium sulfate crystals usually contain various organic and inorganic impurities derived from the various types of material streams or feedstocks used in their production.

[0003] ε-Caprolactam is an important organic chemical raw material used, inter alia, in the production of polyamide 6 (also called nylon 6). ε-Caprolactam can be prepared by subjecting cyclohexanone oxime to a Beckmann rearrangement reaction in the presence of sulfuric acid and / or oleum (see, for example, Ullmann's Encyclopedia of Industrial Chemistry, (Germany), Wiley-VCH, 2018, chapter "Caprolactam"; https: / / doi.org / 10.1002 / 14356007.a05_031.pub3). Oleum is a mixture of sulfuric acid and SO3 and acts as a catalyst for the conversion of cyclohexanone oxime to ε-caprolactam. After the rearrangement reaction, a base (usually ammonia) is added to the Beckmann rearrangement mixture to yield a neutralized Beckmann rearrangement mixture. The neutralized Beckmann rearrangement mixture typically comprises an aqueous ammonium sulfate phase (referred to herein as the "first aqueous ammonium sulfate phase") and an aqueous ε-caprolactam phase. The first aqueous ammonium sulfate phase further contains some ε-caprolactam, and the aqueous ε-caprolactam phase further contains some ammonium sulfate. In addition, both phases contain unwanted organic components, i.e., impurities. Such unwanted organic components are, for example, cyclohexanone oxime, cyclohexanone, 2-cyclohexen-1-one, 2-hydroxycyclohexan-1-one, 1,2-cyclohexanedione, and 1,2,3,4,6,7,8,9-octahydrophenazine (see, for example, Du et al., "Impurity formation in the Beckmann rearrangement of cyclohexanone oxime to yield ε-caprolactam", Industrial and Engineering Chemistry Research, (USA), 2017, Vol. 56, No. 48, p. 14207-14213).

[0004] The first aqueous ammonium sulfate phase is usually extracted with an organic solvent to remove residual ε-caprolactam. The organic solvent is usually recovered from the thus obtained first aqueous ammonium sulfate phase that contains little ε-caprolactam. The first aqueous ammonium sulfate phase co-produced in the ε-caprolactam production process often contains more than 50% by weight of water and less than 50% by weight of ammonium sulfate. In addition, such a phase contains unwanted organic components derived from the Beckmann rearrangement reaction.

[0005] The aqueous ε-caprolactam phase can also be extracted with an organic solvent, thereby forming an organic phase containing ε-caprolactam and another aqueous ammonium sulfate phase (referred to herein as the "second aqueous ammonium sulfate phase"). Pure ε-caprolactam is recovered from the organic phase containing ε-caprolactam by a series of purification and concentration steps. Residual trace amounts of the organic solvent are usually recovered from the second aqueous ammonium sulfate phase. Furthermore, this second aqueous ammonium sulfate phase contains unwanted organic components derived from the Beckmann rearrangement reaction.

[0006] Both the above-mentioned first and second aqueous ammonium sulfate phases contain organic components that pollute the environment and thus cannot be easily disposed of. However, the treatment of these two phases in a wastewater treatment plant is very costly because both the ammonia and organic components contained in the phases are at high concentrations.

[0007] The organic components contained in the first and second aqueous ammonium sulfate phases obtained from the neutralized Beckmann rearrangement mixture make it difficult to produce pure crystalline ammonium sulfate by evaporation crystallization from the aqueous ammonium sulfate phase. During evaporation crystallization, the organic components become concentrated in the aqueous phase and easily mix into the formed ammonium sulfate crystals. To reduce the incorporation of organic components into the ammonium sulfate crystals, a purge is usually carried out, whereby the mother liquor, i.e., the aqueous ammonium sulfate phase in which the organic components are concentrated, is continuously or periodically discharged from the evaporation crystallization section. Due to the fact that the above purge contains organic components, it cannot be discarded as such. As a result, treatment at the cost of the above purge in a wastewater treatment plant is necessary. Furthermore, a further drawback of carrying out the purge from the evaporation crystallization section is that valuable ammonium sulfate is also removed. This reduces the total yield of ammonium sulfate crystals available from the two aqueous ammonium sulfate phases of the neutralized Beckmann rearrangement mixture. Thus, the implementation of the purge has an adverse effect on the economics of ammonium sulfate crystal production from the neutralized Beckmann rearrangement mixture.

[0008] There has long been a need to carry out a method for treating the first and second aqueous ammonium sulfate phases containing organic components obtained from a neutralized Beckmann rearrangement mixture in an economical and ecologically (environmentally) considerate manner.

[0009] Chinese Patent No. 1023790 describes a method for treating the ammonium sulfate solution phase of a neutralized Beckmann rearrangement mixture. Ammonium sulfate crystals are recovered from the ammonium sulfate solution phase, and the organic components derived from the ammonium sulfate solution phase are oxidized by a wet oxidation method. The ammonium sulfate crystals are recovered from the ammonium sulfate solution phase using a crystallization apparatus that functions by evaporation. The mother liquor containing the organic components is taken out from the crystallization apparatus and treated by a wet oxidation method. In the wet oxidation method, the mother liquor is brought into contact with an oxygen-containing gas in a wet oxidation reactor, resulting in gaseous oxidation products and a purified ammonium sulfate solution. The purified ammonium sulfate solution with a reduced amount of organic components is reused in the crystallization apparatus. Chinese Patent No. 1023790 does not disclose the treatment of an elution phase containing ammonium sulfate formed by extracting the aqueous ε-caprolactam phase of a neutralized Beckmann rearrangement mixture with an organic solvent. Therefore, Chinese Patent No. 1023790 does not solve the disposal problem for either type of aqueous ammonium sulfate phase obtainable from a neutralized Beckmann rearrangement mixture. Furthermore, Chinese Patent No. 1023790 does not disclose a method for producing high-purity crystalline ammonium sulfate from an aqueous phase containing ammonium sulfate and organic components.

[0010] JP Patent No. 49032716 discloses a method for treating a neutralized Beckmann rearrangement mixture. After separation of the ε-caprolactam phase, the aqueous ammonium sulfate solution is heat-treated at 150 to 350 °C under pressure in oxygen or a gas containing oxygen. The method of JP Patent No. 49032716 does not disclose the treatment of an elution phase containing ammonium sulfate formed by extracting the aqueous ε-caprolactam phase of a neutralized Beckmann rearrangement mixture with an organic solvent. Therefore, JP Patent No. 49032716 also does not solve the disposal problem for either type of aqueous ammonium sulfate phase obtainable from a neutralized Beckmann rearrangement mixture. Furthermore, JP Patent No. 49032716 does not disclose a method for producing high-purity crystalline ammonium sulfate from an aqueous phase containing ammonium sulfate and organic components.

[0011] EP 1206411 discloses a method for treating a mixture, in particular a neutralized Beckmann rearrangement mixture, comprising an ammonium sulfate solution phase containing a first organic component and an aqueous ε-caprolactam phase containing a second organic component, the method comprising forming an aqueous liquid containing the first and second organic components and subjecting the aqueous liquid to a wet oxidation process for purifying the aqueous liquid. EP 1206411 does not disclose the production of a liquid oily phase. Further, EP 1206411 does not disclose a method for producing high-purity crystalline ammonium sulfate from an aqueous phase containing ammonium sulfate and an organic component.

[0012] Prior art methods use disadvantageous wet oxidation, which requires the combination of high pressure and high temperature and usually uses pure oxygen as a feedstock. The severe process conditions consume large amounts of energy and generate high processing costs. In addition, the uncertain processing costs are high, especially due to the consumption of pure oxygen. Furthermore, the investment and maintenance costs for the plant equipment used in wet oxidation technology are very high due to the severe process conditions used. Finally, the technology using humid air does not always completely oxidize all materials to carbon dioxide and water. Rather, some intermediate compounds, such as carboxylic acids, corresponding to about one quarter of the original mass of the organic component, are formed. For this reason, the liquid waste treated by wet oxidation requires additional treatment steps for final purification.

[0013] [Summary of the Invention] Based on the prior art, the object of the present invention is - a first aqueous ammonium sulfate phase contaminated with organic components, directly obtained from a neutralized Beckmann rearrangement mixture, and - a second aqueous ammonium sulfate phase contaminated with organic components, obtained from the processing of the aqueous ε-caprolactam phase of a neutralized Beckmann rearrangement mixture to provide a method for obtaining and / or treating.

[0014] At the same time, an object of the present invention is to provide a method for producing high-purity ammonium sulfate crystals from at least one aqueous ammonium sulfate phase contaminated with organic components. Furthermore, an object of the present invention is to provide an environmentally friendly method in that a small amount of liquid waste containing organic components is generated and / or utilized for another purpose. It is also an object of the present invention to provide a method with high economic value and high cost efficiency.

[0015] Overall, an object of the present invention is to provide an economical and environmentally friendly method for producing high-purity ammonium sulfate from an initial Beckmann rearrangement mixture containing organic components.

[0016] The foregoing objects are solved by the method according to claim 1, the plant according to claim 13, the crystalline ammonium sulfate according to claim 14, and the liquid oily phase according to claim 15.

[0017] In a first aspect, the present invention is a method for producing crystalline ammonium sulfate in an industrial-scale plant, the plant comprising a Beckmann rearrangement reaction section, a neutralization section, a first liquid-liquid separation section, first and second extraction sections, a first solvent recovery section, first and second evaporative crystallization sections, and a first solid-liquid separation section and comprising the method comprising a) components (i) sulfuric acid and / or oleum; and (ii) cyclohexanone oxime being introduced into the Beckmann rearrangement reaction section and reacted to form a mixture containing ε-caprolactam; b) discharging the resulting mixture containing ε-caprolactam from the Beckmann rearrangement reaction section to the neutralization section; c) adding ammonia and water to a mixture containing ε-caprolactam in a neutralization section, thereby obtaining a neutralized Beckmann rearrangement mixture comprising a first aqueous ammonium sulfate phase, which contains organic components as impurities, and an aqueous ε-caprolactam phase; d) separating the first aqueous ammonium sulfate phase and the aqueous ε-caprolactam phase obtained in the neutralization section in a first liquid-liquid separation section; e) the first aqueous ammonium sulfate phase obtained in step d), e.1) extracting the first aqueous ammonium sulfate phase with a first organic solvent in a first extraction section, thereby obtaining a phase containing the first organic solvent and ε-caprolactam and the first aqueous ammonium sulfate phase after extraction; e.2) feeding the first aqueous ammonium sulfate phase after extraction into a first solvent recovery section, where the first organic solvent is recovered and the first aqueous ammonium sulfate phase after recovery is obtained; e.3) feeding the first aqueous ammonium sulfate phase after recovery into a first evaporative crystallization section and performing evaporative crystallization in this section to obtain crystalline ammonium sulfate and mother liquor in the first evaporative crystallization section, where the mother liquor is an aqueous ammonium sulfate phase in which the organic components are concentrated compared to the first aqueous ammonium sulfate phase after recovery entering the first evaporative crystallization section; e.4) discharging the mother liquor from the first evaporative crystallization section; e.5) discharging a slurry containing crystalline ammonium sulfate from the first evaporative crystallization section and feeding this into a first solid-liquid separation section to recover crystalline ammonium sulfate for treatment; f) the aqueous ε-caprolactam phase obtained in step d), f.1) A step of extracting the aqueous ε-caprolactam phase with a second organic solvent in a second extraction section, whereby a phase containing the second organic solvent and the ε-caprolactam phase and a second aqueous ammonium sulfate phase containing organic components are obtained The step of treating by comprises (i) The mother liquor discharged from the first evaporation crystallization section in step e.4), and / or (ii) The second aqueous ammonium sulfate phase containing organic components obtained in step f.1) being fed into a second evaporation crystallization section, and the evaporation crystallization is carried out such that a three-phase system (1) A liquid oily phase containing organic components; (2) A crystalline ammonium sulfate phase; and (3) A liquid phase containing aqueous ammonium sulfate is formed, (iii) Recovering at least the oily phase from the three-phase system characterized by a method

[0018] In the method of the present invention, ε-caprolactam is produced by a Beckmann rearrangement reaction (step a)) in which cyclohexanone oxime is used as a starting material and sulfuric acid and / or oleum act as catalysts. The Beckmann rearrangement reaction mixture is subsequently neutralized with ammonia (steps b) and c)), resulting in a neutralized Beckmann rearrangement reaction mixture. This neutralized Beckmann rearrangement mixture has two phases, namely a first aqueous ammonium sulfate phase and an aqueous ε-caprolactam phase, both of which contain organic components as impurities. The two phases of the neutralized Beckmann rearrangement mixture are separated from each other in step d), whereby both phases are further processed in steps e) and f).

[0019] The treatment of the first aqueous ammonium sulfate phase in step e) includes extraction using a first organic solvent and obtaining the recovered first aqueous ammonium sulfate phase by recovering the first solvent (steps e.1 and e.2)), and the recovered first aqueous ammonium sulfate phase is fed into a first evaporative crystallization section (step e.3)). In the first evaporative crystallization section, evaporative crystallization is carried out to obtain crystalline ammonium sulfate and mother liquor. A part of the mother liquor is discharged from the first evaporative crystallization section (step e.4)). The crystalline ammonium sulfate obtained in the first evaporative crystallization section is discharged as a slurry and fed into a first solid-liquid separation section to recover the crystalline ammonium sulfate (step e.5)). The processing step e) of the present invention produces high-purity (i.e., pure and large-sized) crystalline ammonium sulfate.

[0020] The treatment of the aqueous ε-caprolactam phase in step f) includes extraction of the aqueous ε-caprolactam phase using a second organic solvent in a second extraction section (step f.1)). By this extraction, a second aqueous ammonium sulfate phase containing organic components is produced. In addition, a phase containing ε-caprolactam and the second organic solvent is obtained, and pure ε-caprolactam is recovered from this phase.

[0021] The method of the present invention is such that (i) the mother liquor discharged in step e.4), or (ii) the second aqueous ammonium sulfate phase containing organic components obtained in step f.1), or both, are fed into a second evaporative crystallization section, where evaporative crystallization is carried out so that (1) a liquid oily phase containing organic components; (2) a crystalline ammonium sulfate phase; and (3) a liquid phase containing aqueous ammonium sulfate are formed (i.e., the conditions in the second evaporative crystallization section are set so as to be like that).

[0022] At least the liquid oily phase is recovered from the three-phase system. This means that the oily phase is obtained as another product of the process of the present invention separately from ammonium sulfate crystals and ε-caprolactam. The liquid oily phase of the process of the present invention can be advantageously utilized for other uses as further described below.

[0023] Surprisingly, the process of the present invention produces high-quality ε-caprolactam and high-quality crystalline ammonium sulfate while reducing the discharge of impurities. This is achieved by carrying out the reuse loop (i) and / or (ii) as outlined above, and by performing a second evaporative crystallization on the reuse stream to produce a liquid oily phase containing organic impurities.

[0024] The production of the liquid oily phase is an advantage of the present invention. By producing the liquid oily phase, the problem of liquid waste is reduced or even eliminated by producing a low-volume by-product in the form of a liquid oily phase containing organic components. This liquid oily phase can be used for other uses such as combustion itself or combustion with heat recovery. The liquid oily phase can be incinerated without the need to add additional fuel. The heat recovered from combustion is advantageously used to generate steam. Therefore, the present invention further provides, as another product, a useful oily phase as defined herein, in particular a liquid oily phase - with 0.5 to 25% by weight, preferably 1 to 20% by weight, and more preferably 2 to 15% by weight of ε-caprolactam; - with an amount of organic components of 500 to 2000 grams COD / kg, preferably 750 to 2000 grams COD / kg, and more preferably 1000 to 2000 grams COD / kg relative to the liquid oily phase and provides a liquid oily phase containing the same.

[0025] The present invention further includes crystalline ammonium sulfate produced in a second evaporative ammonium sulfate crystallization section in the presence of the liquid oily phase.

[0026] Unlike prior art methods, the method of the present invention does not require harsh processing conditions such as high overpressure (e.g., above 2 bar) or temperatures above 120 °C. The method of the present invention does not require pure oxygen as a feedstock. All of this is advantageous as it reduces the energy consumption for carrying out the method of the present invention and also reduces the investment and maintenance costs for the plant equipment required to implement the method of the present invention. However, harsh processing conditions were common in the prior art, particularly in wet oxidation.

[0027] Overall, the method of the present invention produces high-quality products in the form of ammonium sulfate crystals and ε-caprolactam, and additionally a liquid oily phase. The method of the present invention can be operated at low cost since it consumes little energy and reduces the production of liquid waste containing organic components that need to be treated in a wastewater treatment plant. As a result, the overall carbon footprint for the production process of ε-caprolactam and ammonium sulfate is improved.

[0028] Next to the method of the present invention, the present invention further provides an industrial-scale plant suitable for carrying out the method of the present invention. The plant comprises, as equipment, at least those described above for the method of the present invention, namely a Beckmann rearrangement reaction section, a neutralization section, a first liquid-liquid separation section, first and second extraction sections, a first solvent recovery section, first and second evaporative crystallization sections, and a first solid-liquid separation section.

[0029] Advantageous embodiments of the present invention are set out in the dependent claims and are explained in more detail below.

[0030] [Detailed Description of the Invention] [Method] The method of the present invention is carried out in an industrial-scale plant comprising a Beckmann rearrangement reaction section, a neutralization section, a first liquid-liquid separation section, first and second extraction sections, a first solvent recovery section, first and second evaporative crystallization sections, and a first solid-liquid separation section.

[0031] The method of the present invention is a multi-stage method in which several phases are obtained and further processed. The term "phase" as used herein refers to a distinguishable substance such as a solid, liquid or gas. The liquid phase as used herein can contain organic components and / or inorganic components in dissolved and / or undissolved states in a solvent. The "aqueous phase" as used herein means that most of the solvent is water. "Solvent" is a liquid in which the "product" (e.g., ε-caprolactam or ammonium sulfate) is at least partially dissolved as used herein. "Solvent" and "product" refer to different entities. For example, an aqueous ε-caprolactam phase may contain 30% by weight of water and 70% by weight of ε-caprolactam. Since most of the solvent (100% in this example) is water, this is called "aqueous". The "organic phase" as used herein means that most of the solvent is an organic solvent.

[0032] The phases obtained by the method of the present invention differ in their contained components, or at least in the amount of the contained components even if the types of the contained components are the same. The ranges shown below for each component of a particular phase can be combined with any range shown for another component of the same phase. In particular, ranges of the same "preferred level" are compatible for different components of the same phase.

[0033] <<Step a)>> In step a) of the method of the present invention, components such as sulfuric acid and / or oleum, and cyclohexanone oxime are introduced into the Beckmann rearrangement reaction section. Cyclohexanone oxime is subjected to a Beckmann rearrangement reaction in which sulfuric acid and / or oleum acts as a catalyst. The Beckmann rearrangement reaction is highly exothermic and is therefore typically controlled by a cooling system. The reaction mixture produced in the Beckmann rearrangement section contains ε-caprolactam.

[0034] <<Step b)>> In step b) of the method of the present invention, the Beckmann rearrangement reaction mixture containing ε-caprolactam is discharged from the Beckmann rearrangement reaction section to the neutralization section. As used herein, the term "discharge" means "removal".

[0035] <<Step c)>> Step c) of the method of the present invention is carried out in the neutralization section. In step c), ammonia and water are added to the Beckmann rearrangement reaction mixture containing ε-caprolactam. Thereby, a neutralized Beckmann rearrangement mixture is formed. The neutralized Beckmann rearrangement mixture contains two phases. One phase is an aqueous ammonium sulfate phase. This is referred to herein as the "first aqueous ammonium sulfate phase". The second phase is an aqueous ε-caprolactam phase. Both phases of the neutralized Beckmann rearrangement mixture contain impurities in the form of organic components. In addition, both phases may contain inorganic components as impurities. The aqueous ε-caprolactam phase contains ammonium sulfate in particular as an inorganic impurity.

[0036] As used herein, the term "organic component" refers to an organic compound that can be oxidized. These oxidizable organic compounds correspond to unwanted impurities in the phase containing the compound. The type and amount of the organic component may vary among the various phases generated during the method of the present invention. For example, the first aqueous ammonium sulfate phase generated in step c) contains ε-caprolactam as the largest amount of organic component or impurity.

[0037] <<Step d)>> In step d) of the method of the present invention, the first aqueous ammonium sulfate phase and the aqueous ε-caprolactam phase of the neutralized Beckmann rearrangement mixture are separated from each other in the first liquid-liquid separation section.

[0038] In an embodiment of the present invention, the first aqueous ammonium sulfate phase in step d) contains 25 to 50% by weight, preferably 35 to 48% by weight, and more preferably 39 to 45% by weight of ammonium sulfate.

[0039] In another embodiment of the present invention, the first aqueous ammonium sulfate phase in step d) contains ε-caprolactam as the main organic component. The first aqueous ammonium sulfate phase may further contain cyclohexanone oxime and by-products of the Beckmann rearrangement reaction, such as cyclohexanone, 2-cyclohexen-1-one, 2-hydroxycyclohexan-1-one, 1,2-cyclohexanedione, and 1,2,3,4,6,7,8,9-octahydrophenazine as additional organic components.

[0040] As used herein, the term "main organic component" means that the organic component occupies the largest proportion with respect to the amount expressed in g COD / kg for a specific phase. The acronym "COD" stands for "Chemical Oxygen Demand" and is used herein to indicate the amount of oxidizable organic components in a given phase. Thus, the acronym "COD" is used herein to describe the content of oxidizable organic components in a given phase. The COD value is measured in accordance with ASTM D 1252-95 (dichromate method).

[0041] In another embodiment of the present invention, the aqueous ε-caprolactam phase in step d) contains 50 to 85% by weight, preferably 55 to 80% by weight, and more preferably 65 to 76% by weight of ε-caprolactam.

[0042] In a further embodiment of the present invention, the aqueous ε-caprolactam phase in step d) contains ammonium sulfate. In yet another embodiment of the present invention, the aqueous ε-caprolactam phase in step d) contains 0.1 to 3% by weight, preferably 0.15 to 1.5% by weight, and more preferably 0.3 to 1.0% by weight of ammonium sulfate.

[0043] In another embodiment of the present invention, the aqueous ε-caprolactam phase in step d) contains, as impurities (separate from ε-caprolactam), organic components in an amount of 1 to 40, preferably 2 to 25, and more preferably 4 to 15 g COD / kg with respect to the aqueous ε-caprolactam phase.

[0044] <<Step e)>> In step e) of the method of the present invention, the first aqueous ammonium sulfate phase obtained in step d) is treated. Step e) includes several sub-steps.

[0045] In step e.1) of the method of the present invention, extraction of the first aqueous ammonium sulfate phase with the first organic solvent in the first extraction section is carried out. By extraction, a phase containing the first organic solvent and ε-caprolactam and the first aqueous ammonium sulfate phase after extraction are produced. This extraction step has the advantage that ε-caprolactam is almost completely removed from the first aqueous ammonium sulfate phase. The extracted ε-caprolactam can be fed to a concentration step and a purification step for producing primary ε-caprolactam. At the same time, the first aqueous ammonium sulfate phase after extraction has almost lost ε-caprolactam as an impurity. In an embodiment of the present invention, the first aqueous ammonium sulfate phase after extraction substantially does not contain ε-caprolactam. "Substantially does not contain ε-caprolactam" means that the first aqueous ammonium sulfate phase after extraction contains 0.005 to 0.3% by weight, preferably 0.01 to 0.1% by weight, and more preferably 0.02 to 0.05% by weight of ε-caprolactam.

[0046] In an embodiment of the present invention, the first organic solvent is an aromatic hydrocarbon, a halogenated hydrocarbon, a C4-C 10 aliphatic alcohol and / or alicyclic alcohol. In a further embodiment of the present invention, the first organic solvent is selected from the group consisting of benzene, toluene, chloroform, trichloroethane, 4-methyl-2-pentanol, 1-octanol, 2-ethylhexanol, and mixtures thereof. As another example, the first organic solvent is selected from the group consisting of benzene, toluene, trichloroethylene, alcohol, and mixtures thereof. In yet another embodiment, benzene and toluene are preferred first organic solvents. All of the above definitions and preferred subgroups also independently apply to the second organic solvent further described below.

[0047] In step e.2) of the method of the present invention, the first aqueous ammonium sulfate phase after extraction is fed into the first solvent recovery section, where the first organic solvent and the aqueous ammonium sulfate phase are recovered. The recovered aqueous ammonium sulfate phase still contains organic components as impurities, but most of the first organic solvent is removed. This recovery step has the advantage that the first organic solvent can be reused in step e.1). In an embodiment of the present invention, the organic component content of the aqueous ammonium sulfate phase in step e.2) is lower than that of the first aqueous ammonium sulfate phase in step d), where the organic component content is represented by COD.

[0048] In step e.3) of the method of the present invention, the recovered first aqueous ammonium sulfate phase is fed into the first evaporative crystallization section where evaporative crystallization is carried out.

[0049] Examples of crystallization apparatuses that can be used in the crystallization section are described in Don W. Green and James O. Maloney, "Perry's Chemical Engineers Handbook", 7th edition, (USA), McGraw Hill, 1997, Section 18, pp. 44-55. The crystallization apparatus is usually operated at a temperature of 20 to 180 °C and a pressure of 2 kPa to 0.8 MPa.

[0050] Ammonium sulfate crystallization by evaporation of the aqueous ammonium sulfate phase as used in this specification typically involves heat input to evaporate water and concentrate the remaining phase. To reduce the steam consumption required for evaporative crystallization in the production of crystals from the aqueous phase, a series of crystallization apparatuses can be integrated with respect to heat input (see, for example, I. Kristjansson, Geothermics, (Netherlands), 1992, Vol. 21, p.765-771). This is also preferred according to the present invention. In a series of heat-integrated crystallization apparatuses, water is boiled in crystallization apparatuses arranged such that each apparatus is maintained at a lower pressure than the previous one. Since the boiling temperature of water decreases with decreasing pressure, the steam generated by boiling in one crystallization apparatus can be used to heat the next apparatus. Only the first crystallization apparatus (operating at the highest pressure) requires an external heat source. This is generally done by passing high-temperature steam through the reboiler of the first crystallization apparatus in the series. The resulting lower-temperature steam is used to heat the next crystallization apparatus, and so on. This means that the series of crystallization apparatuses operate while reducing the temperature. This type of crystallization is also called multiple-effect crystallization.

[0051] The recovered first ammonium sulfate phase fed to the first evaporative crystallization apparatus in step e.3) contains 25 to 50% by weight, preferably 35 to 48% by weight, and more preferably 39 to 45% by weight of ammonium sulfate.

[0052] The organic component content of the recovered first ammonium sulfate phase fed to the first evaporative crystallization apparatus in step e.3) is 0.02 to 20 grams COD / kg, preferably 0.05 to 15 grams COD / kg, and more preferably 0.1 to 10 grams COD / kg based on the recovered first ammonium sulfate phase.

[0053] In the first evaporative crystallization section, crystalline ammonium sulfate and mother liquor are obtained by evaporative crystallization. The mother liquor is an aqueous ammonium sulfate phase in which the organic components are concentrated as compared with the first aqueous ammonium sulfate phase after recovery that enters the first evaporative crystallization section. Usually, the mother liquor is also concentrated with respect to ammonium sulfate.

[0054] In step e.4) of the method of the present invention, the mother liquor is discharged from the first evaporative crystallization section. The discharge of a part of the mother liquor is also called a purge. The purge in step e.4) has the advantage of reducing the organic component content of the mother liquor remaining in the first evaporative crystallization section. By keeping the organic component content in the mother liquor remaining in the first evaporative crystallization section low, the quality of the ammonium sulfate crystals formed is improved, that is, the coloring of the ammonium sulfate crystals is reduced, the organic component content is reduced, and the production of large crystals is promoted. The lower limit value of the organic component content in the mother liquor of the first evaporative crystallization section is not decisive and is mainly determined by the economy of the method since more mother liquor needs to be purged. The mother liquor can be withdrawn from the crystallization apparatus by any means well known to those skilled in the art. For example, the clarified liquid may be purged from the clarification zone of the crystallization apparatus, it is also possible that the mother liquor removed as the overflow of the liquid cyclone is purged, and / or it is also possible that the mother liquor is removed through the outlet of the mother liquor circulation line. When the first evaporative crystallization section operates as a multiple-effect crystallization, the mother liquor can be discharged from any stage of the multiple-effect crystallization. Preferably, the mother liquor is discharged from the final stage of the multiple-effect crystallization.

[0055] In an embodiment of the present invention, the flow rate of the mother liquor purged from the first evaporative crystallization section is 0.5 to 30 parts by volume per unit time, preferably 1 to 25 parts by volume per unit time, more preferably 2 to 20 parts by volume per unit time, and accordingly the flow rate of the recovered first aqueous ammonium sulfate phase supplied to the first evaporative crystallization section is 100 parts by volume per unit time. When the flow rate of the mother liquor withdrawn from the first evaporative crystallization apparatus decreases, the content of organic components in the mother liquor generally increases assuming that the settings of other flows, particularly the flow rate of the recovered first aqueous ammonium sulfate phase supplied to the first evaporative crystallization section, remain unchanged. When the supply of the recovered first aqueous ammonium sulfate phase to the first evaporative crystallization apparatus and / or the purge of the mother liquor from the first evaporative crystallization apparatus are carried out discontinuously or batchwise, the aforementioned flow rate is of course the average supply amount or withdrawal amount per unit time.

[0056] The mother liquor discharged from the first evaporative crystallization apparatus in step e.4) usually contains 35 to 60% by weight, preferably 38 to 55% by weight, and more preferably 42 to 52% by weight of ammonium sulfate.

[0057] The content of organic components in the mother liquor discharged from the first evaporative crystallization section is usually 5 to 150 g COD / kg, preferably 10 to 75 g COD / kg, and more preferably 15 to 50 g COD / kg based on the mother liquor.

[0058] In step e.5) of the method of the present invention, a slurry containing crystalline ammonium sulfate is discharged from the first evaporative crystallization section and fed into a first solid-liquid separation section to recover the crystalline ammonium sulfate.

[0059] As used herein, the term "slurry" refers to an aqueous phase containing solids. The ammonium sulfate slurry contains ammonium sulfate crystals, dissolved ammonium sulfate, water, and impurities. Typically, the impurities present in the ammonium sulfate slurry are organic components. In addition, inorganic components may also be present.

[0060] In an embodiment of the present invention, step e.5) is e.5.1) a step of discharging a slurry containing crystalline ammonium sulfate from a first evaporative crystallization section and introducing the slurry into a first solid-liquid separation section; e.5.2) a step of discharging crystalline ammonium sulfate from the first solid-liquid separation section and introducing the crystalline ammonium sulfate into a first drying section, wherein dried crystalline ammonium sulfate is obtained and includes.

[0061] The ammonium sulfate crystals obtained in step e.5) contain moisture and impurities in addition to ammonium sulfate itself. Typically, the impurities in the ammonium sulfate crystals are organic components. In addition, inorganic components may also be present. Such inorganic components may be calcium salts, iron salts, and silicon-containing compounds.

[0062] Larger ammonium sulfate crystals are generally preferred because they have greater economic value. In an embodiment of the present invention, the ammonium sulfate crystals produced in step e.5) have an average median diameter exceeding 0.8 mm. Preferably, the average median diameter of the ammonium sulfate crystals produced in step e.5) is 1.0 mm to 4.0 mm.

[0063] It is desirable to produce high-purity (almost) white crystalline ammonium sulfate for various applications. (Almost) white high-purity ammonium sulfate crystals have higher economic value than brown ammonium sulfate crystals with a higher impurity concentration. Typically, the color of high-purity crystalline ammonium sulfate is white, off-white, or light yellow. The ammonium sulfate crystals produced in step e.5) of the method of the present invention are high-purity crystals by continuous processing of steps e.1) to e.5). In an embodiment of the present invention, the color of the ammonium sulfate crystals produced in step e.5) is white to off-white.

[0064] The formation of colored ammonium sulfate crystals containing impurities in the form of organic components is more pronounced during evaporation crystallization at higher temperatures. Therefore, it is advantageous to crystallize the ammonium sulfate phase containing organic components without using too high a temperature. This limits the temperature range in which evaporation crystallization can be carried out. To avoid operating the crystallization apparatus at high temperatures, the number of continuously operable crystallization apparatuses is restricted. The temperature at which a given ammonium sulfate phase can be crystallized without forming colored crystals and without entraining visible solid organic components depends on both the amount and composition of the organic components contained in that given ammonium sulfate phase. As a result, when attempting to crystallize two or more ammonium sulfate phases with different organic component compositions, different maximum temperatures can be used for each phase in the crystallization apparatus. Preferably, the evaporation crystallization apparatus for crystallizing ammonium sulfate from an ammonium sulfate phase containing organic components is operated at a temperature lower than 120 °C. More preferably, the evaporation crystallization apparatus is operated at a temperature lower than 110 °C.

[0065] ≪Step f)≫ In step f) of the process according to the invention, the aqueous ε-caprolactam phase obtained in step d) is treated. Step f) comprises one or more sub-steps.

[0066] In step f.1) of the process according to the invention, the aqueous ε-caprolactam phase is extracted with a second organic solvent in a second extraction section, thereby obtaining a phase containing the second organic solvent and ε-caprolactam and a second aqueous ammonium sulfate phase containing organic components. This aqueous ammonium sulfate phase is referred to throughout this disclosure as the "second aqueous ammonium sulfate phase" in order to distinguish it from the first aqueous ammonium sulfate phase obtained directly from the Beckmann rearrangement mixture neutralized in step d).

[0067] In an embodiment of the invention, the second aqueous ammonium sulfate phase contains 0.1 to 10% by weight, preferably 0.2 to 8% by weight, and more preferably 0.5 to 6% by weight of ammonium sulfate.

[0068] In a further embodiment of the present invention, the second aqueous ammonium sulfate phase in step f.1) has an organic component content of 5 to 200 grams COD / kg, preferably 15 to 150 grams COD / kg, and more preferably 20 to 100 grams COD / kg, based on the second aqueous ammonium sulfate phase.

[0069] In another embodiment of the present invention, the second aqueous ammonium sulfate phase in step f.1) contains cyclohexanone oxime as an organic component, by-products of the Beckmann rearrangement reaction such as cyclohexanone, 2-cyclohexen-1-one, 2-hydroxycyclohexan-1-one, 1,2-cyclohexanedione, and 1,2,3,4,6,7,8,9-octahydrophenazine, and a second organic solvent.

[0070] The first aqueous ammonium sulfate phase in step d) and the second aqueous ammonium sulfate phase in step f.1) have in common that they contain ammonium sulfate, water, and an organic component as an impurity. The first and second aqueous ammonium sulfate phases may further contain an inorganic component as an impurity. Typically, the first and second aqueous ammonium sulfate phases differ from each other at least in the concentration of the contained organic component and ammonium sulfate. In one embodiment of the present invention, the second aqueous ammonium sulfate phase in step f.1) has a higher organic component content than the recovered first aqueous ammonium sulfate phase obtained in step e2), where the organic component content is represented by COD. In another or further embodiment of the present invention, the second aqueous ammonium phase in step f.1) has a lower ammonium sulfate content than the first aqueous ammonium sulfate phase in step d), where the ammonium sulfate content is represented by weight%.

[0071] The extraction of the aqueous ε-caprolactam phase in step d) is carried out using a second organic solvent. The second organic solvent can be selected independently of the first organic solvent from the same list of solvents as defined above for the first organic solvent. In practice, it is particularly advantageous if the same organic solvent is used as the first and second organic solvents.

[0072] In another embodiment of the present invention, after the second organic solvent is recovered from the second aqueous ammonium sulfate phase containing the organic component obtained in step f.1), the second aqueous phase containing the obtained organic component is introduced into a second evaporative crystallization section. This additional step will be referred to as f.2), which has the advantage that the second organic solvent is reused from the second aqueous ammonium sulfate phase and can thus be reused in step f.1). Another advantage of recovering the second organic solvent from the second aqueous ammonium sulfate phase is that the second aqueous ammonium sulfate phase contains less of the second organic solvent as an impurity.

[0073] Separate from the second aqueous ammonium sulfate phase, the phase containing the second organic solvent and ε-caprolactam obtained in the second extraction section in step f.1) is further processed. Several concentration and purification steps for producing primary ε-caprolactam starting from the phase containing the second organic solvent and ε-caprolactam in step f.1) are well known to those skilled in the art. The chapter on "Caprolactam" in "Ullmann's Encyclopedia of Industrial Chemistry", 2018 (https: / / doi.org / 10.1002 / 14356007.a05_031.pub3) describes several (industrial) processes for producing such primary ε-caprolactam. Furthermore, WO 02 / 070475 describes a method for recovering and purifying ε-caprolactam from an organic solvent. WO 02 / 070475 envisions the following steps as possible work-ups also for the phase containing ε-caprolactam of the present invention, namely: a) washing the solution with water or an aqueous alkaline solution to obtain a washed solution containing ε-caprolactam and the organic solvent and a washing residue; b) evaporating the organic solvent from the washed solution to obtain an ε-caprolactam product; c) optionally, hydrogenating the ε-caprolactam product; d) optionally, evaporating water from the ε-caprolactam product; e) distilling the ε-caprolactam product to recover ε-caprolactam and a distillation residue; f) extracting the distillation residue with an organic solvent in the presence of water to obtain (i) an extract containing ε-caprolactam dissolved in the organic solvent and (ii) an aqueous effluent; and g) recycling the extract to step a) or b).

[0074] ≪Second Evaporative Crystallization≫ In the method of the present invention, the second evaporative crystallization is carried out in a so-called second evaporative crystallization section. The second type of crystallization section is charged with (i) the mother liquor discharged from the first evaporative crystallization section in step e.4), and / or (ii) the second aqueous ammonium sulfate phase containing the organic components obtained in step f.1).

[0075] In an embodiment of the present invention, after water is removed from the second aqueous ammonium sulfate phase containing the organic components obtained in step f.1), the resulting concentrated second aqueous ammonium sulfate phase is charged into the second evaporative crystallization section.

[0076] The second evaporative crystallization in the second type of crystallization section is carried out to produce the following three-phase system, namely (1) a liquid oily phase containing organic components; (2) a crystalline ammonium sulfate phase; and (3) a liquid phase containing aqueous ammonium sulfate is carried out so as to occur.

[0077] The term "is carried out so as to" means that the second evaporative crystallization section is operated under conditions that enable the formation of a three-phase system. In particular, since a very large amount of water is evaporated from the aqueous feedstock in the second evaporative crystallization section, the aqueous ammonium sulfate phase becomes supersaturated with respect to both ammonium sulfate and organic components, and as a result, the liquid oily phase (1) containing organic components and the crystalline ammonium sulfate phase (2) are formed separately from the liquid phase (3) containing aqueous ammonium sulfate. The specific operating conditions of the second evaporative crystallization section include that the organic component content and the weight fraction of ammonium sulfate in the liquid phase containing aqueous ammonium sulfate in the second evaporative crystallization section are at least 100 g COD / kg and at least 5 g COD / kg, respectively, with respect to the mother liquor of the liquid phase containing aqueous ammonium sulfate, and that 5 g COD / kg of the liquid phase containing aqueous ammonium sulfate is derived from ε-caprolactam, and that the ammonium sulfate is at least 36% by weight.

[0078] The main difference between the first and second evaporative crystallization carried out in step e.3) is that there is no outflow of the mother liquor in the second evaporative crystallization. Evaporation of water in the second evaporative crystallization results in a more concentrated mother liquor, but the organic impurities can no longer dissolve in the mother liquor, leading to the formation of not only ammonium sulfate crystals but also a liquid oily phase containing organic impurities. Thus, the main difference between the first and second evaporative crystallization sections is the concentration of organic impurities: in the first evaporative crystallization section, ammonium sulfate becomes supersaturated with respect to ammonium sulfate and ammonium sulfate crystals are formed as a separate phase, but the mother liquor remains unsaturated with respect to organic components (by purging a portion of the mother liquor containing impurities) and no separate liquid oily phase is formed.

[0079] Next, in terms of evaporation conditions, the flow of the feedstock to the first and second evaporative crystallization sections also differs from each other. In an embodiment of the present invention, the ammonium sulfate feedstock (i) and / or (ii) to the second evaporative crystallization apparatus has a higher organic component content than the recovered first aqueous ammonium sulfate phase introduced into the first evaporative crystallization section in step e.3), where the organic component content is expressed as COD. In a further embodiment, the ammonium sulfate content of the ammonium sulfate feedstock (i) and / or (ii) to the second evaporative crystallization apparatus is lower than the ammonium sulfate content of the recovered first aqueous ammonium sulfate phase introduced into the first evaporative crystallization section in step e.3), where the ammonium sulfate content is expressed as weight %. In particular, the ammonium sulfate content of the ammonium sulfate feedstock (ii) to the second evaporative crystallization apparatus is lower than the ammonium sulfate content of the recovered first aqueous ammonium sulfate phase introduced into the first evaporative crystallization section in step e.3), where the ammonium sulfate content is expressed as weight %.

[0080] The second evaporative crystallization according to the present invention produces in particular a liquid oily phase, which is at least recovered from the three-phase system in step (iii).

[0081] As used herein, the term "recovered" means that the substance is isolated in the form of a product, is spared from loss, and / or is utilized for another use.

[0082] In an embodiment of the present invention, the liquid oily phase contains ε-caprolactam, an organic component, ammonium sulfate, and water. The liquid oily phase is formed by evaporating a very large amount of water during the second evaporative crystallization so that the organic component becomes supersaturated, and by becoming no longer soluble in the aqueous ammonium sulfate-containing liquid phase in the second evaporative crystallization section.

[0083] At least the liquid oily phase is recovered from the three-phase system produced in the second evaporative crystallization section. For this purpose, a plant suitable for carrying out the method of the present invention can further comprise a second liquid-liquid separation section and a second solid-liquid separation section, whereby the recovery of at least the liquid oily phase in step (iii) is (iii.1) discharging a mixture containing the liquid oily phase and the liquid phase containing aqueous ammonium sulfate from the second evaporative crystallization section and introducing the mixture into a second liquid-liquid separation section where the two phases are separated; (iii.2) recovering the separated liquid oily phase and the liquid phase containing separated aqueous ammonium sulfate from the second liquid-liquid separation section; (iii.3) discharging a slurry containing crystalline ammonium sulfate from the second evaporative crystallization section and introducing the slurry into a second solid-liquid separation section where the crystalline ammonium sulfate and the liquid aqueous ammonium sulfate phase are recovered separately is included.

[0084] Thus, by the method of the present invention, ammonium sulfate crystals can be obtained from the first aqueous ammonium sulfate phase in step d) and the second aqueous ammonium sulfate phase in step f.1).

[0085] In a further embodiment of the present invention, the weight ratio of the liquid oily phase to the liquid phase containing aqueous ammonium sulfate in the mixture containing the liquid oily phase and the liquid phase containing aqueous ammonium sulfate, which is discharged from the second evaporative crystallization section in step (iii.1) and introduced into the second liquid-liquid separation section, is less than 2:1, preferably less than 1:1. In another embodiment of the present invention, the weight ratio of the liquid oily phase to the liquid phase containing aqueous ammonium sulfate in the mixture containing the liquid oily phase and the liquid phase containing aqueous ammonium sulfate, which is discharged from the second evaporative crystallization section in step (iii.1) and introduced into the second liquid-liquid separation section, is from 1:0.5 to 1:50, preferably from 1:1 to 1:40, more preferably from 1:2 to 1:30, and most preferably from 1:4 to 1:15. In fact, good results were obtained when the ratio was about 1:10.

[0086] In another embodiment of the present invention, - the liquid phase containing aqueous ammonium sulfate recovered from the second liquid-liquid separation section in step (iii.2), and / or - the liquid aqueous ammonium sulfate phase recovered from the second solid-liquid separation section in step (iii.3) is introduced into the second evaporative crystallization section. This has the advantage that the liquid phase containing aqueous ammonium sulfate recovered from the second liquid-liquid and / or the liquid aqueous ammonium sulfate phase recovered from the second solid-liquid separation section are used to produce crystalline ammonium sulfate in the second evaporative crystallization section. Under such circumstances, the amount of crystalline ammonium sulfate produced in the second evaporative crystallization is maximized in the process of the present invention.

[0087] In another embodiment of the present invention, the crystalline ammonium sulfate discharged from the second solid-liquid separation section in step (iii.3) is washed with an aqueous solution. This is advantageous because impurities associated with the crystalline ammonium sulfate are removed.

[0088] The ammonium sulfate crystals obtained in the second evaporative crystallization section are usually of lower quality than those obtained from the first evaporative crystallization section in terms of the characteristics of size and / or color, and / or the caking behavior. However, the low-quality ammonium sulfate crystals also have uses (for example, as fertilizers for less demanding spreading methods, such as hand-spreading methods), or can be discharged as waste. The main purpose of the second evaporative crystallization section according to the present invention is the production of a liquid oily phase with the advantages outlined above. In a particularly useful embodiment of the present invention, the crystalline ammonium sulfate obtained from the second evaporative crystallization section in step (i) is subsequently fed into a dissolution section, where the crystalline ammonium sulfate is dissolved in water, thereby obtaining a phase containing aqueous ammonium sulfate to be fed into the first evaporative crystallization section. This has the advantage that a fraction producing potentially lower-quality ammonium sulfate crystals is processed to yield a liquid oily phase containing most of the impurities and a crystalline ammonium sulfate phase that can be dissolved and reused in the first evaporative crystallization section, resulting in higher-purity ammonium sulfate crystals being obtained overall in the method of the present invention.

[0089] In an embodiment of the present invention, the liquid oily phase has a density (measured at a temperature of 25 °C) in the range of 1100 to 1275 kg / m 3 , preferably 1125 to 1250 kg / m 3 , and most preferably 1150 to 1225 kg / m 3 . This has the advantage of obtaining a fairly concentrated liquid oily phase, i.e., an oily phase with a high organic content, low water and ammonium sulfate contents, and that can burn easily.

[0090] In a further embodiment of the present invention, the liquid oily phase contains 0.5 to 25% by weight, preferably 1 to 20% by weight, more preferably 2 to 15% by weight of ε-caprolactam, and has an organic component content of 500 to 2000 grams COD / kg, preferably 750 to 2000 grams COD / kg, more preferably 1000 to 2000 grams COD / kg with respect to the liquid oily phase. The aforementioned ranges for ε-caprolactam and organic components in the liquid oily phase can be combined with each other in any way. In particular, the ranges of the same "preferred level" are compatible.

[0091] In another embodiment, the liquid oily phase contains 1 to 30% by weight, preferably 2 to 25% by weight, more preferably 5 to 20% by weight of ammonium sulfate.

[0092] The liquid oily phase obtained by the method of the present invention can be used for other purposes. In an embodiment of the present invention, the liquid oily phase is used as fuel in an incinerator. In another embodiment, the liquid oily phase is used as fuel in an incinerator, and at least a part of the heat generated in the incinerator is used for evaporation of moisture.

[0093] <Ammonium sulfate crystals> The present invention further relates to crystalline ammonium sulfate produced in an evaporative ammonium sulfate crystallization section in the presence of a liquid oily phase and an aqueous liquid phase containing ammonium sulfate as described above with respect to the second evaporative crystallization section.

[0094] The ammonium sulfate crystals produced in the evaporative ammonium sulfate crystallization section in the presence of a liquid oily phase and an aqueous liquid phase containing ammonium sulfate preferably have an average median diameter exceeding 0.6 mm. More preferably, the average median diameter of the ammonium sulfate crystals produced in the evaporative ammonium sulfate crystallization section in the presence of a liquid oily phase and an aqueous liquid phase containing ammonium sulfate is 0.8 mm to 3.0 mm.

[0095] In another embodiment of the present invention, ammonium sulfate crystals produced in the presence of a liquid oily phase and a liquid phase containing aqueous ammonium sulfate, separated by filtration, and washed with water have an organic component content of 0.2 to 250 grams of COD / kg, preferably 1 to 50 grams of COD / kg, and most preferably 3 to 15 grams of COD / kg based on crystalline ammonium sulfate.

[0096] In a further embodiment of the present invention, the color of ammonium sulfate crystals produced in an evaporative ammonium sulfate crystallization section in the presence of a liquid oily phase and an aqueous ammonium sulfate liquid phase is (light) brown.

[0097] <Liquid oily phase> The present invention further relates to a liquid oily phase, - 0.5 to 25% by weight, preferably 1 to 20% by weight, and more preferably 2 to 15% by weight of ε-caprolactam, and - an organic component in an amount of 500 to 2000 grams of COD / kg, preferably 750 to 2000 grams of COD / kg, and more preferably 1000 to 2000 grams of COD / kg based on the liquid oily phase and relates to a liquid oily phase containing the same.

[0098] Regarding the characteristics and use of the liquid oily phase, the same applies as already described for the method of the present invention.

[0099] <Plant> The present invention further relates to an ammonium sulfate crystallization plant for producing crystalline ammonium sulfate from an aqueous stream containing organic components, wherein the aqueous stream containing organic components is derived from an ε-caprolactam production process.

[0100] The plant of the present invention is an industrial-scale plant comprising at least a Beckmann rearrangement reaction section, a neutralization section, a first liquid-liquid separation section, first and second extraction sections, a first solvent recovery section, first and second evaporative crystallization sections, and a first solid-liquid separation section, and is a plant configured to carry out the method of the present invention.

[0101] The processing capacity of the ammonium sulfate crystallization plant is typically selected based on the amount of ammonium sulfate phase discharged from the plant for the production of ε-caprolactam.

[0102] The production capacity of ammonium sulfate crystals in the industrial-scale ammonium sulfate plant of the present invention is on the order of thousands of tons per year (kilotons per year; kta). In an embodiment of the present invention, the production capacity of ammonium sulfate crystals in the industrial-scale plant exceeds 10,000 tons per year (10 kta). Preferably, the production capacity of ammonium sulfate crystals in the industrial-scale plant is 100 kta to 2,000 kta. More preferably, the production capacity of ammonium sulfate crystals in the industrial-scale plant is 150 kta to 1,500 kta.

[0103] Hereinafter, the present invention will be described with reference to the drawings, which show certain embodiments of the present invention. However, the present invention is as defined in the claims and generally as described herein. The present invention should not be limited to the embodiments shown in the following figures for illustrative purposes.

Brief Description of the Drawings

[0104]

Figure 1

Figure 2

Figure 3

Figure 4

[0105] Figure 1 shows an alternative mode of the method of the invention.

[0106] In Figure 1A, the first aqueous ammonium sulfate phase [α] obtained from the neutralized Beckmann rearrangement mixture is processed and used for evaporative crystallization in the first evaporative crystallization section [I]. During evaporative crystallization, crystalline ammonium sulfate [γ] and mother liquor are obtained. The crystalline ammonium sulfate [γ] is recovered. The mother liquor is an aqueous ammonium sulfate phase in which the organic components are concentrated compared to the processed first aqueous ammonium sulfate phase entering the first evaporative crystallization section [I]. A portion of the mother liquor is discharged from the first evaporative crystallization section [I] to the second evaporative crystallization section [a]. The second aqueous ammonium sulfate phase [β] generated by extraction of the aqueous ε-caprolactam phase obtained from the neutralized Beckmann rearrangement mixture is not fed into the second evaporative crystallization section [a]. In the second evaporative crystallization section, evaporative crystallization is carried out so that a three-phase system is formed. The three-phase system includes a liquid oily phase, a crystalline ammonium sulfate phase, and a liquid phase containing aqueous ammonium sulfate. At least the liquid oily phase [δ] is recovered.

[0107] In Figure 1B, the first aqueous ammonium sulfate phase [α] obtained from the neutralized Beckmann rearrangement mixture is processed and used for evaporative crystallization in the first evaporative crystallization section [I]. During evaporative crystallization, crystalline ammonium sulfate [γ] and mother liquor are obtained. Optionally, a portion of the mother liquor is discharged from the first evaporative crystallization section [I] to the outside of the process (not shown). The crystalline ammonium sulfate [γ] is recovered. The second aqueous ammonium sulfate phase [β] generated by extraction of the aqueous ε-caprolactam phase obtained from the neutralized Beckmann rearrangement mixture is fed into the second evaporative crystallization section [a]. In the second evaporative crystallization section, evaporative crystallization is carried out so that a three-phase system is formed. The three-phase system includes a liquid oily phase, a crystalline ammonium sulfate phase, and a liquid phase containing aqueous ammonium sulfate. At least the liquid oily phase [δ] is recovered.

[0108] Figure 1C is a combination of alternative modes of the method of the present invention described in Figures 1A and 1B. In Figure 1C, a portion of the mother liquor of the first evaporative crystallization section [I] is fed into the second evaporative crystallization section [a]. In addition, a second aqueous ammonium sulfate phase [β] is fed into the second evaporative crystallization section [a].

[0109] In Figure 2, the following two different ammonium sulfate-containing feedstocks

[0101] and

[0102] , namely the mother liquor

[0101] with concentrated organic components purged from the ammonium sulfate crystallization section, and the second aqueous ammonium sulfate phase

[0102] containing organic components obtained as an effluent from the extraction of the crude aqueous ε-caprolactam phase using an organic solvent, are fed into one or more evaporative crystallizers (e.g., Oslo type (fluidized bed) crystallizer, draft tube baffle (DTB) crystallizer, forced circulation crystallizer) of the evaporative crystallization section [a]. Optionally, before being fed into one or more evaporative crystallizers of the evaporative crystallization section [a], the second aqueous ammonium sulfate phase

[0102] containing organic components is fed into a solvent recovery section where the solvent is recovered (not shown). Optionally, before being fed into one or more evaporative crystallizers of the evaporative crystallization section [a], the second aqueous ammonium sulfate phase

[0102] containing organic components is fed into a pre-concentration section where mainly water is removed, for example, by evaporation and / or membrane filtration (not shown).

[0110] The water evaporated in one or more evaporative crystallizers of the evaporative crystallization section [a] is condensed and discharged as condensate

[0103] . Generally, the condensate

[0103] is disposed of as waste and treated in a wastewater treatment system (not shown). Optionally, the condensate

[0103] is reused in the Beckmann rearrangement reaction, optionally after purification treatment (e.g., adsorption treatment with activated carbon) (not shown).

[0111] In one or more evaporative crystallization apparatuses in the evaporative crystallization section [a], the concentration of the organic component and the concentration of ammonium sulfate are very high, resulting in a three-phase system, namely, a liquid oily phase with a high organic component content, a liquid phase containing aqueous ammonium sulfate, and a crystalline ammonium sulfate phase.

[0112] A mixture

[0104] containing the liquid oily phase and the liquid phase containing aqueous ammonium sulfate is discharged from one or more evaporative crystallization apparatuses in the evaporative crystallization section [a] and introduced into the liquid-liquid separation section [b]. This mixture may also contain a small amount of the crystalline ammonium sulfate phase. The liquid-liquid separation section [b] is composed of one or more separation vessels, where, for example, the liquid oily phase with a high organic component content separates as the top layer, and the liquid phase containing aqueous ammonium sulfate (and optionally the crystalline ammonium sulfate phase) separates as the bottom layer.

[0113] The liquid oily phase

[0106] is recovered from the liquid-liquid separation section [b]. Optionally, this liquid oily phase

[0106] is introduced into an incinerator (not shown). The liquid oily phase may further be stored and / or used or sold as fuel. Optionally, a part of the heat generated by burning the liquid oily phase in the incinerator is used to generate steam (not shown).

[0114] The liquid aqueous ammonium sulfate phase

[0105] (and optionally the solid phase containing ammonium sulfate crystals) is also discharged from the liquid-liquid separation section [b] and introduced into one or more evaporative crystallization apparatuses in the evaporative crystallization section [a].

[0115] The ammonium sulfate-containing slurry [

[0108] ], which contains a liquid phase containing aqueous ammonium sulfate and ammonium sulfate crystals, is discharged from one or more evaporative crystallization apparatuses in the evaporative crystallization section [a] and fed into one or more solid-liquid separation apparatuses (e.g., (continuous) filters, centrifuges, decanters, elutriation columns, salt legs, hydrocyclones, or combinations thereof) in the solid-liquid separation section [c]. In the solid-liquid separation section [c], the ammonium sulfate wet crystals [

[0110] ] are separated from the liquid phase containing aqueous ammonium sulfate. Optionally, the ammonium sulfate wet crystals are then washed, whereby the ammonium sulfate wet crystals are washed (preferably with water or an aqueous ammonium sulfate phase) to reduce the content of organic components (adhering to the surface) of the ammonium sulfate wet crystals (not shown). The remaining aqueous ammonium sulfate phase or dilute ammonium sulfate slurry [

[0109] ] is fed back into one or more evaporative crystallization apparatuses [a].

[0116] The ammonium sulfate wet crystals [

[0110] ] are dried in one or more drying apparatuses (e.g., a fluidized bed dryer) in the drying section [d], whereby water vapor [

[0120] ] and dry ammonium sulfate crystals [

[0121] ] are obtained and discharged separately. Optionally, sieving, coating, and / or blending with other compounds are performed on the dry ammonium sulfate crystals [

[0121] ] (not shown).

[0117] In Figure 3, an aqueous ammonium sulfate phase

[0201] containing organic components obtained as an effluent from the extraction of a crude aqueous ε-caprolactam phase using an organic solvent is fed into a pre-concentration section [f] where water is mainly removed. Preferably, the pre-concentration section [f] comprises a reverse osmosis membrane and / or one or more evaporators, such as a falling-film type, natural convection type, forced convection type, rising-film type, rising-film plate type, or a combination thereof. Optionally, before being fed into the pre-concentration section [f], the aqueous ammonium sulfate phase

[0201] containing organic components is fed into a solvent recovery section where the solvent is recovered (not shown). In the pre-concentration section [f], water is separated and discharged as a water stream

[0202] . Generally, the water stream

[0202] is disposed of as waste and treated in a wastewater treatment system (not shown). Optionally, the water stream

[0202] is reused in the Beckmann rearrangement reaction, optionally after purification treatment (e.g., adsorption treatment with activated carbon) (not shown). Optionally, if the water stream

[0202] is in the vapor phase (e.g., as a result of evaporation), the condensation heat may be recovered (not shown).

[0118] The post-pre-concentration effluent

[0203] is discharged from the pre-concentration section [f] and fed into one or more evaporative crystallization apparatuses (e.g., an Oslo type (fluidized bed type) crystallization apparatus, a draft tube baffle (DTB) crystallization apparatus, a forced circulation type crystallization apparatus) in the ammonium sulfate crystallization section [a]. Optionally, for pH adjustment, ammonia (water) is fed into the post-pre-concentration effluent

[0203] prior to feeding into one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a] (not shown). In addition, an aqueous ammonium sulfate phase

[0204] containing organic components purged from another ammonium sulfate crystallization section is fed into one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a].

[0119] In one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a], the water evaporated is condensed and discharged as condensed water

[0205] . Generally, the condensed water

[0205] is disposed of as waste and treated in a wastewater treatment system (not shown). Optionally, the condensed water

[0205] is reused in a process for producing ammonium sulfate and ε-caprolactam based on cyclohexanone oxime produced from cyclohexanone after optional purification treatment (e.g., adsorption treatment with activated carbon) (not shown).

[0120] The concentration of the organic components and the concentration of ammonium sulfate in one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a] are very high, resulting in a three-phase system, namely a liquid oily phase with a high organic component content, a liquid phase containing aqueous ammonium sulfate, and a crystalline ammonium sulfate phase.

[0121] A mixture

[0206] mainly containing a liquid oily phase with a high organic component content and a liquid phase containing aqueous ammonium sulfate is discharged from one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a] and fed into the liquid-liquid separation section [b]. This mixture may also contain a small amount of the crystalline ammonium sulfate phase. The liquid-liquid separation section [b] consists of one or more separation vessels, where the liquid oily phase with a high organic component content separates as the top layer and the liquid phase containing aqueous ammonium sulfate separates as the bottom layer. Optionally, the bottom layer also contains a very small portion (<4% by weight) of the crystalline ammonium sulfate phase.

[0122] The top layer is recovered from the liquid-liquid separation section [b] as the liquid oily phase

[0208] . Optionally, this liquid oily phase

[0208] is fed into an incinerator (either on-site or optionally off-site) (not shown). Optionally, a part of the heat generated by burning the liquid oily phase in the incinerator is used to generate steam (not shown).

[0123] The lower layer, which is a liquid phase containing aqueous ammonium sulfate (optionally containing a crystalline ammonium sulfate phase), is discharged as an aqueous phase

[0207] from the liquid-liquid separation section [b] and introduced into one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a].

[0124] The ammonium sulfate-containing slurry

[0209] is discharged from one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a] and introduced into one or more solid-liquid separation apparatuses (e.g., (continuous) filters, centrifuges, decanters, elutriation columns, salt legs, liquid cyclones, or combinations thereof) in the solid-liquid separation section [c]. In the solid-liquid separation section [c], ammonium sulfate wet crystals

[0211] are separated. Optionally, a washing step for the ammonium sulfate wet crystals is included, whereby the ammonium sulfate wet crystals are washed (not shown) (preferably with water or an aqueous ammonium sulfate phase) to reduce the content of organic components (adhering to the surface) of the ammonium sulfate wet crystals.

[0125] The remaining liquid phase containing aqueous ammonium sulfate or the dilute ammonium sulfate slurry

[0210] is reintroduced into one or more evaporative crystallization apparatuses in the ammonium sulfate crystallization section [a].

[0126] The ammonium sulfate wet crystals

[0211] and the aqueous phase

[0212] are introduced into the ammonium sulfate dissolution section [e]. Preferably, the aqueous phase

[0212] contains more than 90% by weight of water. Optionally, the aqueous phase

[0212] contains ammonium sulfate. Generally, the ammonium sulfate dissolution section [e] is composed of one or more containers including a mixing compartment and an overflow compartment. An aqueous stream

[0213] of ammonium sulfate is discharged from the ammonium sulfate dissolution section [e]. Preferably, the aqueous stream

[0213] of ammonium sulfate is an aqueous phase containing about 40% by weight of ammonium sulfate but no solid ammonium sulfate crystals. Preferably, the aqueous stream

[0213] of ammonium sulfate is introduced as a feedstock into the ammonium sulfate crystallization section, where crystalline ammonium sulfate, condensed water, and purge are produced.

[0127] In Figure 4, cyclohexanone oxime [1] and oleum [2] are fed into the Beckmann rearrangement section [A]. Cyclohexanone oxime may be produced by various techniques. One option is the HPO (registered trademark) process in which cyclohexanone oxime is formed by reacting hydroxylamine obtained by hydrogenation of nitrate with cyclohexanone. Another option for producing cyclohexanone oxime is the ammoximation process in which cyclohexanone oxime is formed by reacting ammonia, hydrogen peroxide and cyclohexanone. A further option for producing cyclohexanone oxime is the Raschig process in which cyclohexanone oxime is formed by reacting hydroxylamine obtained by reduction of nitrite with cyclohexanone.

[0128] The Beckmann rearrangement mixture [3] and the aqueous ammonia phase [4] are added to the neutralization section [B] to produce a neutralized Beckmann rearrangement mixture [5]. As another example, the aqueous ammonia phase [4] is replaced by the individual addition of water and gaseous ammonia (not shown). A neutralization mixture containing an aqueous ammonium sulfate phase and an aqueous crude ε-caprolactam phase is obtained in the neutralization section [B]. The aqueous ammonium sulfate phase contains ε-caprolactam and organic components as impurities. The aqueous crude ε-caprolactam phase contains ammonium sulfate and organic components as impurities.

[0129] The neutralization mixture [5] is fed into the liquid-liquid separation section [C], where the aqueous ammonium sulfate phase and the aqueous crude ε-caprolactam phase are separated from each other by phase separation. An aqueous crude ε-caprolactam phase [6] and an aqueous ammonium sulfate phase [7] exit the liquid-liquid separation section [C].

[0130] The aqueous ammonium sulfate phase [7] is fed into the extraction section [F], where the solvent [8] is added and the dissolved ε-caprolactam is recovered. Various solvents can be used, such as benzene, toluene, trichloroethylene, and alcohols, especially 1-octanol and 2-ethylhexanol and mixtures of alcohols. The extraction section [F] may comprise one or more extraction devices, such as (countercurrent) extraction columns and mixer settlers. A mixture [9] containing ε-caprolactam and the solvent exits the extraction section [F].

[0131] The obtained aqueous ammonium sulfate phase

[10] after extraction is fed into the solvent recovery section [G], where the solvent is recovered from the aqueous ammonium sulfate phase

[10] after extraction and released as the recovered solvent

[11] . The solvent recovery section [G] may comprise one or more distillation columns and / or one or more steam strippers.

[0132] After the solvent is recovered, the aqueous ammonium sulfate phase

[12] is adjusted in pH in the pH adjustment section [H] by adding a pH modifier

[13] (preferably ammonia or sulfuric acid), whereby an aqueous ammonium sulfate phase

[14] after pH adjustment is obtained. The aqueous ammonium sulfate phase

[14] after pH adjustment is fed into one or more evaporation crystallization apparatuses in the evaporation crystallization section [I] (for example, an Oslo type (fluidized bed type) crystallization apparatus, a draft tube baffle (DTB) crystallization apparatus, or a forced circulation type crystallization apparatus). In order to reduce the energy consumption required for the evaporation of water in the ammonium sulfate crystallization apparatus, a vapor recompression method and / or a multiple-effect evaporation method may be applied. The aqueous stream of ammonium sulfate discharged from the ammonium sulfate dissolution section [e]

[0213] is fed into one or more evaporation crystallization apparatuses in the evaporation crystallization section [I]. The water evaporated in one or more evaporation crystallization apparatuses in the evaporation crystallization section [I] is condensed and discharged as condensed water

[15] . Optionally, the condensed water

[15] is reused in the Beckmann rearrangement reaction (not shown) after optional purification treatment (for example, adsorption treatment with activated carbon). A purge

[0204] is discharged from one or more evaporation crystallization apparatuses in the evaporation crystallization section [I]. The purge

[0204] is a mother liquor, which is an aqueous ammonium sulfate phase having a higher organic component content compared to the organic component content of the fed aqueous ammonium sulfate phase

[14] after pH adjustment. The purge

[0204] is fed into the second evaporation crystallization section [a].

[0133] An ammonium sulfate-containing slurry

[16] is discharged from one or more evaporation crystallization apparatuses in the evaporation crystallization section [I] and fed into one or more solid-liquid separation apparatuses [J] (for example, a (continuous) filter, a centrifuge, a centrifugal sedimentation machine, a hydraulic classifier, a salt leg, a liquid cyclone, or a combination thereof), whereby wet ammonium sulfate crystals

[18] are separated. The remaining aqueous ammonium sulfate phase or dilute ammonium sulfate slurry

[17] is fed into one or more evaporation crystallization apparatuses in the evaporation crystallization section [I].

[0134] Optionally, a washing step of the ammonium sulfate wet crystals is included, whereby the ammonium sulfate wet crystals are washed (not shown), preferably with water or an aqueous ammonium sulfate phase, in order to reduce the content of organic components (adhering to the surface) of the ammonium sulfate wet crystals.

[0135] The ammonium sulfate wet crystals

[18] are dried in one or more drying apparatuses [K] (e.g., a fluidized bed dryer), whereby water vapor

[19] and dry ammonium sulfate crystals 20 are released. Optionally, for the dry ammonium sulfate crystals

[20] , sieving, coating, and / or blending with other compounds are carried out (not shown).

[0136] In the extraction section [L], the aqueous crude ε-caprolactam [6] is extracted with a solvent

[21] , whereby an aqueous effluent

[22] and a mixture containing ε-caprolactam and the solvent 24 are obtained. Various solvents, such as benzene, toluene, trichloroethylene, and alcohols, especially 1-octanol and 2-ethylhexanol and mixtures thereof, can be used. The extraction section [L] can comprise one or more extraction apparatuses, such as (countercurrent) extraction columns and mixer settlers. The obtained aqueous effluent

[22] is fed into the solvent recovery section [M].

[0137] In the solvent recovery section [M], the solvent is recovered from the aqueous effluent

[22] and released as the recovered solvent

[23] . The solvent recovery section [M] may comprise one or more distillation columns and / or one or more steam strippers. The aqueous effluent

[0201] resulting after solvent recovery is fed to a preliminary concentration section [f], where mainly water is removed. Preferably, the preliminary concentration section [f] comprises a reverse osmosis membrane and / or one or more evaporators, such as a falling film type, natural convection type, forced convection type, rising film type, rising film plate type, or a combination thereof. In the preliminary concentration section [f], water is separated and released as a water stream

[0202] . Generally, the water stream

[0202] is disposed of as waste and treated in a wastewater treatment system (not shown). Optionally, the water stream

[0202] is reused in the Beckmann rearrangement reaction, optionally after a purification treatment (e.g., adsorption treatment with activated carbon) (not shown). Optionally, if the water stream

[0202] is in the vapor phase (e.g., as a result of evaporation), the condensation heat may also be recovered (not shown).

[0138] For the mixture

[24] containing ε-caprolactam and the solvent discharged from the extraction section [L], further work-up is carried out in the ε-caprolactam purification and concentration section [N], whereby the recovered solvent

[25] and purified ε-caprolactam

[26] are obtained. In industrial practice, various embodiments of the ε-caprolactam purification and concentration section [N] are realized. Generally, these embodiments may consist of a combination of apparatuses for extraction, hydrogenation, ion exchange, crystallization, pH adjustment, and / or (vacuum) distillation.

[0139] For the sections [a], [b], [c], and [e] in Figure 4 and the various streams entering and leaving these sections, it is as described above for Figure 3.

Examples

[0140] The following examples serve to explain the present invention in more detail, particularly certain forms of the present invention. However, the examples are not intended to limit the present disclosure.

[0141] The COD content described in the following examples is measured according to the dichromate method in accordance with ASTM D 1252-95.

[0142] The examples and comparative examples were carried out in an industrial-scale continuous operation plant that produces both ε-caprolactam and crystalline ammonium sulfate and has an annual production capacity of ε-caprolactam exceeding 200 kta.

[0143] <Comparative Example 1> Comparative Example 1 is very similar to the embodiment of the present invention shown in FIG. 4. However, in this comparative example, the aqueous effluent

[0201] after benzene recovery was introduced into the wastewater treatment system. In addition, the purge

[0204] was also introduced into the wastewater treatment system.

[0144] In the caprolactam industrial plant, cyclohexanone oxime was produced from cyclohexanone by the HPO (registered trademark) method. Cyclohexanone oxime [1] was converted to ε-caprolactam using oleum [2] in a three-stage Beckmann rearrangement reaction section [A]. The resulting Beckmann rearrangement mixture [3] (ε-caprolactam sulfate in an excess of sulfuric acid) was then neutralized with aqueous ammonia [4] in a neutralization section [B], thereby obtaining a neutralization mixture [5] of aqueous crude ε-caprolactam and an aqueous ammonium sulfate phase. The above two phases were separated into aqueous crude ε-caprolactam [6] and an aqueous ammonium sulfate phase [7] in a liquid-liquid separation section [C].

[0145] The aqueous crude ε-caprolactam [6] was extracted with benzene

[21] in the countercurrent extraction column of the extraction section [L], whereby an ε-caprolactam mixture

[24] containing benzene and an aqueous effluent

[22] were obtained. For the ε-caprolactam mixture

[24] containing benzene, further work-up was carried out in the ε-caprolactam purification and concentration section [N], whereby recovered benzene

[25] and primary ε-caprolactam

[26] were obtained. The primary ε-caprolactam

[26] was sold as a raw material for nylon 6 production. The aqueous effluent

[22] was fed to the distillation column of the benzene recovery section [M], from which benzene

[23] was recovered as the overhead product, and the aqueous effluent

[0201] after benzene recovery was obtained as the bottom product. The aqueous effluent

[0201] after benzene recovery was composed of about 4 wt% ammonium sulfate and about 4 wt% organic components, and the remainder was mainly water. In Comparative Example 1, the aqueous effluent

[0201] after benzene recovery was fed into the wastewater treatment system, where most of the ammonium components and organic components were removed.

[0146] The aqueous ammonium sulfate phase [7] was fed into the upper part of the countercurrent extraction column of the extraction section [F], and benzene [8] was fed into the lower part. A mixture [9] containing ε-caprolactam containing benzene and the aqueous ammonium sulfate phase

[10] after extraction were discharged from the extraction section [F]. The aqueous ammonium sulfate phase

[10] after extraction was fed to the distillation column of the benzene recovery section [G], from which benzene

[11] was recovered as the overhead product, and the aqueous ammonium sulfate phase

[12] after benzene recovery was obtained as the bottom product. The aqueous ammonium sulfate phase

[12] after solvent recovery was pH-adjusted to a pH value of about 5 (measured at a temperature of 25 °C) by adding aqueous ammonia

[13] in the pH adjustment section [H], whereby the aqueous ammonium sulfate phase

[14] after pH adjustment was obtained.

[0147] This aqueous ammonium sulfate phase after pH adjustment

[14] was fed into three Oslo - type crystallizers using the multiple - effect evaporation method in the evaporative crystallization section [I]. The water evaporated in the evaporative crystallization section [I] was condensed and discharged as condensed water

[15] . In Comparative Example 1, the purge

[0204] was discharged from the evaporative crystallization section [I]. The purge

[0204] was then fed into a wastewater treatment system where most of the ammonium components and organic components were removed. The purge

[0204] was composed of water, approximately 40 wt% ammonium sulfate, and organic impurities (about 40 grams COD / kg with respect to the purge).

[0148] A slurry

[16] containing ammonium sulfate was discharged from the evaporative crystallization section [I] and fed into a centrifuge in the solid - liquid separation section [J]. In this section, after washing with water, ammonium sulfate wet crystals

[18] were separated. The remaining ammonium sulfate phase

[17] containing some fine ammonium sulfate crystals was fed back into the evaporative crystallization section [I]. Finally, the ammonium sulfate wet crystals

[18] were dried in a fluidized - bed dryer in the drying section [K], thereby discharging water vapor

[19] and dry ammonium sulfate crystals

[20] . The dry ammonium sulfate crystals

[20] were sieved and then sold as high - purity crystalline ammonium sulfate. The dry ammonium sulfate crystals

[20] had an average median diameter of approximately 3 mm and were slightly white in color.

[0149] <Example 1> Example 1 describes an embodiment of the present invention as shown in FIG. 4.

[0150] A repetition of the experiment described in Comparative Example 1, with the difference that in this example, the aqueous effluent

[0201] after purge

[0204] and benzene recovery is not fed into the wastewater treatment system, but into a device equipped with a preliminary concentration section [f], an ammonium sulfate crystallization section [a], a liquid-liquid separation section [b], a solid-liquid separation section [c], and an ammonium sulfate dissolution section [e]. From this device, the aqueous stream

[0213] of the ammonium sulfate phase was fed into the first ammonium sulfate crystallization section [I]. From the liquid-liquid separation section [b], a liquid oily phase

[0208] having a high organic component content was discharged and burned in an incinerator.

[0151] The aqueous effluent

[0201] after benzene recovery, consisting of approximately 4 wt% ammonium sulfate, approximately 4 wt% organic matter, and the remainder mainly water, was fed into the vapor-heated falling-film evaporator of the preliminary concentration section [f], whereby a part of the water was evaporated and discharged as vapor

[0202] .

[0152] The resulting outflow after preliminary concentration

[0203] consists of approximately 8% by weight of ammonium sulfate, approximately 8% by weight of organic components, and the remainder mainly composed of water. It is discharged from the falling-film evaporator in the preliminary concentration section [f], and after ammonia water is first introduced for pH adjustment, it is fed into the Oslo crystallizer in the ammonium sulfate crystallization section [a]. In addition, the purge

[0204] discharged from the evaporative crystallization section [I] is fed into the Oslo crystallizer in the ammonium sulfate crystallization section [a]. The vapor

[0202] is supplied to the heat exchanger of the Oslo crystallizer in the ammonium sulfate crystallization section [a]. The water is evaporated in the Oslo crystallizer in the ammonium sulfate crystallization section [a] and condensed and discharged as condensed water

[0205] . The temperature of the Oslo crystallizer in the ammonium sulfate crystallization section [a] is maintained at approximately 70°C. The concentrations of the organic components and ammonium sulfate in the Oslo crystallizer in the ammonium sulfate crystallization section [a] are very high, forming a three-phase system, namely a liquid oily phase with a high organic component content, a liquid phase containing aqueous ammonium sulfate, and a solid phase containing ammonium sulfate crystals.

[0153] A mixture of a liquid oily phase with a high organic component content and a liquid phase containing aqueous ammonium sulfate

[0206] was discharged as a side stream from an Oslo crystallizer in an ammonium sulfate crystallization section [a] and introduced into a separation vessel in a liquid-liquid separation section [b]. This mixture contained a small amount of fine ammonium sulfate crystals. The weight-to-weight ratio of the liquid oily phase with a high organic component content to the liquid phase containing aqueous ammonium sulfate in the mixture

[0206] was on average 1:8. Phase separation was observed in the separation vessel in the liquid-liquid separation section [b], whereby the liquid oily layer became the top layer. The average residence time in the separation vessel in the liquid-liquid separation section [b] was about 1 hour. The top layer was discharged as a liquid oily phase

[0208] with a high organic component content from the separation vessel in the liquid-liquid separation section [b]. A typical composition of this liquid oily phase

[0208] was an organic component content (expressed as COD) of 1000 - 1500 grams / kg relative to the liquid oily phase; ε-caprolactam of 3 - 6 wt%; ammonium sulfate of 12 - 20 wt%; and water of 18 - 30 wt%. The density (measured at a temperature of 25 °C) of this liquid oily phase

[0208] was 1150 - 1220 kg / m 3 for the liquid oily phase. The lower layer, which was the phase containing aqueous ammonium sulfate, was discharged as an aqueous phase

[0207] from the separation vessel in the liquid-liquid separation section [b] and introduced into the Oslo crystallizer in the ammonium sulfate crystallization section [a].

[0154] The ammonium sulfate-containing slurry

[0209] was discharged from the Oslo crystallizer in the ammonium sulfate crystallization section [a] and introduced into a continuous centrifuge in a solid-liquid separation section [c], where ammonium sulfate crystals were separated. The ammonium sulfate crystals were washed with water and then discharged as washed ammonium sulfate wet crystals

[0211] . The remaining aqueous phase containing ammonium sulfate, i.e., the dilute ammonium sulfate slurry

[0210] , was re-introduced into the Oslo crystallizer in the ammonium sulfate crystallization section [a].

[0155] The washed ammonium sulfate wet crystals [

[0211] ] and water [

[0212] ] were fed into the stirring vessel of the ammonium sulfate dissolution section [e]. An aqueous stream [

[0213] ] of an ammonium sulfate phase containing about 40% by weight of ammonium sulfate was discharged from the stirring vessel of the ammonium sulfate dissolution section [e]. The aqueous stream [

[0213] ] of the ammonium sulfate phase was fed as a feedstock into the evaporative crystallization section [I].

[0156] The primary ε-caprolactam

[28] and dry ammonium sulfate crystals produced in Example 1 20 had the same quality as those of Comparative Example 1.

[0157] However, in Example 1, the amount of dry ammonium sulfate crystals 20 per ton of ε-caprolactam produced increased by about 3% compared to Comparative Example 1.

[0158] Comparing the results of Example 1 with those of Comparative Example 1, it has been shown that it is possible to avoid the disposal of the phase containing aqueous ammonium sulfate and organic components without adversely affecting the amount and quality of the ε-caprolactam and ammonium sulfate products obtained. Furthermore, Example 1 shows that 3% more high-purity crystalline ammonium sulfate is obtained. In addition, a liquid oily phase that can be incinerated without adding extra fuel is obtained. All these advantages enable the production of more valuable products, the reduction of costs related to the disposal of by-products, and the reduction of environmental impact. As an overall result, Example 1 reduced the overall carbon footprint of the process for producing polyamide 6 and its co-products.

Claims

1. A method for producing crystalline ammonium sulfate in an industrial-scale plant, the plant comprising: - a Beckmann rearrangement reaction section, - a neutralization section, - a first liquid-liquid separation section, - first and second extraction sections, - a first solvent recovery section, - first and second evaporative crystallization sections, and - a first solid-liquid separation section ; The method comprising: a) feeding components (i) sulfuric acid and / or oleum, and (ii) cyclohexanone oxime to the Beckmann rearrangement reaction section and reacting the components to form a mixture containing ε-caprolactam; b) discharging the obtained mixture containing ε-caprolactam from the Beckmann rearrangement reaction section to the neutralization section; c) adding ammonia and water to the mixture containing ε-caprolactam in the neutralization section, thereby obtaining a neutralized Beckmann rearrangement mixture containing a first aqueous ammonium sulfate phase and an aqueous ε-caprolactam phase, both containing organic components as impurities; d) separating the first aqueous ammonium sulfate phase and the aqueous ε-caprolactam phase obtained in the neutralization section in the first liquid-liquid separation section; e) taking the first aqueous ammonium sulfate phase obtained in step d) and e.1) extracting the first aqueous ammonium sulfate phase with a first organic solvent in the first extraction section, thereby obtaining a phase containing the first organic solvent and ε-caprolactam and the first aqueous ammonium sulfate phase after extraction; e.2) feeding the first aqueous ammonium sulfate phase after extraction to the first solvent recovery section, where the first organic solvent is recovered and the first aqueous ammonium sulfate phase after recovery is obtained; e.3) feeding the first aqueous ammonium sulfate phase after recovery to the first evaporative crystallization section and performing evaporative crystallization in the first evaporative crystallization section to obtain crystalline ammonium sulfate and mother liquor in the first evaporative crystallization section, the mother liquor being an aqueous ammonium sulfate phase in which the organic components are concentrated compared to the first aqueous ammonium sulfate phase after recovery entering the first evaporative crystallization section; e.4) discharging the mother liquor from the first evaporative crystallization section; e.5) Releasing a slurry containing crystalline ammonium sulfate from the first evaporative crystallization section and feeding this into a first solid-liquid separation section to recover the crystalline ammonium sulfate and processing it by the step of; f) The aqueous ε-caprolactam phase obtained in step d) is f.1) A step of extracting the aqueous ε-caprolactam phase with a second organic solvent in a second extraction section, whereby a phase containing the second organic solvent and the ε-caprolactam phase and a second aqueous ammonium sulfate phase containing organic components are obtained, and processing it by this step including (i) The mother liquor discharged from the first evaporative crystallization section in step e.4), and / or (ii) The second aqueous ammonium sulfate phase containing organic components obtained in step f.1 is fed into a second evaporative crystallization section, and the evaporative crystallization is carried out such that the following phases, namely (1) A liquid oily phase containing organic components; (2) A crystalline ammonium sulfate phase; and (3) A liquid phase containing aqueous ammonium sulfate result in a three-phase system being formed, (iii) Recovering at least the liquid oily phase from the three-phase system characterizes the method.

2. The liquid oily phase - contains 0.5 to 25% by weight of ε-caprolactam, - contains 1 to 30% by weight of ammonium sulfate, and - has an organic component content of 500 to 2000 grams COD / kg with respect to the liquid oily phase, The method according to claim 1.

3. The liquid oily phase - contains 1 to 20% by weight of ε-caprolactam, - contains 2 to 25% by weight of ammonium sulfate, and - has an organic component content of 750 to 2000 grams COD / kg with respect to the liquid oily phase, The method according to claim 1.

4. The liquid oily phase - contains 2 to 15% by weight of ε-caprolactam, - contains 5 to 20% by weight of ammonium sulfate, and - has an organic component content of 1000 to 2000 grams COD / kg with respect to the liquid oily phase, The method according to claim 1.

5. The liquid oily phase is used as fuel in an incinerator, The method according to any one of claims 1 to 4.

6. At least part of the heat produced by the incinerator is used for the evaporation of water, The method according to claim 5.

7. The method according to any one of claims 1 to 6, wherein after the second organic solvent is recovered from the second aqueous ammonium sulfate phase containing the organic component obtained in step f.1), the obtained second aqueous phase containing the organic component is introduced into the second evaporative crystallization section.

8. The method according to claim 7, wherein after water is removed from the second aqueous ammonium sulfate phase containing the organic component, the obtained concentrated second aqueous ammonium sulfate phase is introduced into the second evaporative crystallization section.

9. The recovery of crystalline ammonium sulfate from the first evaporative crystallization section in step e.5) e.5.1) discharging a slurry containing crystalline ammonium sulfate from the first evaporative crystallization section and introducing the slurry into the first solid-liquid separation section; e.5.2) discharging crystalline ammonium sulfate from the first solid-liquid separation section and introducing the crystalline ammonium sulfate into the first drying section, whereby dried crystalline ammonium sulfate is obtained The method according to any one of claims 1 to 8, comprising:

10. The plant comprises a second liquid-liquid separation section and a second solid-liquid separation section, and the recovery of at least the liquid oily phase in step (iii) (iii.1) discharging a mixture containing a liquid oily phase and a liquid phase containing aqueous ammonium sulfate from the second evaporative crystallization section and introducing the mixture into a second liquid-liquid separation section where the liquid oily phase and the liquid phase containing aqueous ammonium sulfate are separated; (iii.2) recovering the separated liquid oily phase and the separated liquid phase containing aqueous ammonium sulfate from the second liquid-liquid separation section; (iii.3) discharging a slurry containing crystalline ammonium sulfate from the second evaporative crystallization section and introducing the slurry into a second solid-liquid separation section where crystalline ammonium sulfate and the liquid aqueous ammonium sulfate phase are separately recovered The method according to any one of claims 1 to 9, comprising:

11. - the liquid phase containing aqueous ammonium sulfate recovered from the second liquid-liquid separation section in step (iii.2), and / or - the liquid aqueous ammonium sulfate phase recovered from the second solid-liquid separation section in step (iii.3) is introduced into the second evaporative crystallization section, the method according to claim 10.

12. The method according to claim 10 or 11, wherein the crystalline ammonium sulfate discharged from the second solid-liquid separation section in step (iii.3) is washed with an aqueous solution.

13. The crystalline ammonium sulfate obtained from the second evaporative crystallization section in step (i) is subsequently charged into a dissolution section, where the crystalline ammonium sulfate is dissolved in water, thereby obtaining a phase containing aqueous ammonium sulfate that is charged into the first evaporative crystallization section. The method according to any one of claims 1 to 12.

14. The weight ratio of the liquid oily phase to the liquid phase containing aqueous ammonium sulfate in a mixture containing the liquid oily phase discharged from the second evaporative crystallization section in step (iii.1) and charged into the second liquid-liquid separation section and the liquid phase containing aqueous ammonium sulfate is less than 1. The method according to any one of claims 10 to 13.

15. The first organic solvent used in the first extraction section and the second organic solvent used in the second extraction section are independently selected from the group consisting of benzene, toluene, trichloroethylene, alcohol, and mixtures thereof. The method according to any one of claims 1 to 14.

16. At least - a Beckmann rearrangement reaction section, - a neutralization section, - a first liquid-liquid separation section, - first and second extraction sections, - a first solvent recovery section, - first and second evaporative crystallization sections, and - a first solid-liquid separation section A plant on an industrial scale comprising A plant configured to carry out a method as defined in any one of claims 1 to 15.

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